Heating procedure in a false twist texturizing process

ABSTRACT

An improved heating procedure in a yarn texturizing process by heat setting a false twist therein which comprises confining a sonic flow of superheated steam in closely adjacent surrounding relation along a predetermined longitudinal extent of the yarn continuously moving between the upstream position of twist restraint and the position at which the false twist is imparted to the yarn. The procedure makes possible the texturizing of yarn at production rates of 275 ypm utilizing a steam source at a temperature of 470*F and pressure of 55 psig with a sonic flow confining surface of approximately 6 inches in length and 0.083 inches in diameter.

United States Patent [191 Flanders, Jr.

[ July 23, 1974 1 HEATING PROCEDURE IN A FALSE TWIST TEXTURIZING PROCESS[75] Inventor: Harry E. Flanders, Jr., Greensboro,

[73] Assignee: Burlington Industries, Inc.,

Greensboro, N.C.

22 Filed: Sept. 25, 1972 211 App]. No: 291,585

[52] US. Cl. 57/157 TS, 28/1.4, 28/62, 28/72.12, 57/157 F [51] Int. Cl002g 1/02 [58] Field of Search 57/34 HS, 140 R, 157 F, 57/157 TS, 157 R;28/62, 1.4, 72.12, 72.17

[56] References Cited UNITED STATES PATENTS 3,069,836 12/1962 Dahlstromet a1 57/157 F 3,303,169 2/1967 Pitzl 57/157 F 3,382,658 5/1968 McIntoshet al.... 57/34 l-IS 3,423,809 l/1969 Schmidt 28/72.17 3,447,210 6/1969Barlow 28/62 X 3,457,610 7/1969 Williams et a1. 28/62 X 3,529,413 9/1970Marrinan et a1. 57/140 R 3,608,299 9/1971 Dugas 57/157 F 3,611,69810/1971 Horn 57/140 R 3,640,063 2/1972 Schmid et a1. 57/140 R 3,720,0793/1973 Katsumata et a1. 28/62 X Primary Examiner--John PetrakesAttorney, Agent, or Firm-Cushman, Darby & Cushman [57 ABSTRACT Animproved heating procedure in a yarn texturizing process by heat settinga false twist therein which comprises confining a sonic flow ofsuperheated steam in closely adjacent surrounding relation along apredetermined longitudinal extent of the yarn continuously movingbetween the upstream position of twist restraint and the position atwhich the false twist is imparted to the yarn. The procedure makespossible the texturizing of yarn at production rates of 275 ypmutilizing a steam source at a temperature of 470F and pressure of 55psig with a sonic flow confining surface of approximately 6 inches inlength and 0.083 inches in diameter.

10 Claims, 2 Drawing Figures 1. HEATING PROCEDURE IN A FALSE TWISTTEXTURIZING PROCESS This invention relates to texturizing thermoplasticyarn and more particularly to improved heating procedures for setting afalse twist in thermoplastic yarns.

The practice of texturizing synthetic yarn by heat setting a false twisttherein, which has achieved such widespread use today in connection withpolyester yarns, has been known since at least the 1930s. The US. Patentto Finlayson et al., 2,1 11,211, dated Mar. 15, 1938, discloses anapparatus and procedure for texturizing or crimping synthetic yarn byheat setting a false twist therein, which embodies the basic apparatuscomponents and procedural steps which are still in use today. Theprocedure, as disclosed in the Finlayson et al., patent, involves thefeeding of a supply of thermoplastic yarn continuously into and out of atexturizing or crimping zone within which the yarn is moved past atwisting device. The twisting device serves to build up and maintain inthe yarn passing through the texturizing zone a twist in one directionwhich extends upstream from the twisting device to a first position ofrestraint. The twist in the yarn passing between the first position ofrestraint and the twisting device is heat set therein by elevating andthen reducing the temperature of the yarn before it reaches the twistingdevice. The F inlayson et al., patent discloses the utilization ofsteam, hot air or heated solvent vapors as the setting medium. Insofaras the twist setting structure is concerned, Finlayson et al., disclosethe provision of a main steam pipe to which wet steam is supplied, thepipe having a nipple through which the yarn enters the pipe and anoffset member provided with an outlet nipple through which the yarnpasses out of the steam pipe.

The US. Patent to Kunzle, 2,790,298, dated Apr. 30, 1957, discloses animproved steam heating structure for use in heat setting false twistedthermoplastic yarns which embodies a length of tube of approximately 39inches having labyrinth seals of almost 3 inches in length at each endthereof, the arrangement being operable to withstand and maintain steampressures therein of approximately I atmosphere and up to 5 atmosphereswhile yarn is passed therethrough. I

The machines in use today generally rely upon electricity as a heatsource, utilizing either air or a yarn contacting structure as the yarnheating medium.'These present day heating procedures are the equivalentof those provided in the Finlayson patent disclosure in the sense thatthe heating medium employed to impart the desired heat to the yarn isgenerally maintained in a stationary condition within the system and theyarn is passed thereby. With procedures of this type, the attaining ofthe proper yarn temperature in the process is dependent upon thetemperature of the heating medium and the time period in which theheating medium is in heat exchange relation to the yarn. The heatingtime is, in turn, dependent upon the linear speed of the yarn throughthe processing equipment or the production rate. Experience has shownthat in order to obtain increased production rates, it has beennecessary to extend the length of the heating equipment in order toaccurately control the yarn temperature. In other words, increasedproduction rates could not be accommodated simply by increasing thetemperature of the heating medium. Consequently, the electrical heatersused in most machines today have attained lengths of the order of the 39inch length mentioned above in connection with the Kunzle patent.Contrary to the horizontally inclined disposition of the steam tubesdisclosed in Kunzle, the electrical heaters are disposed vertically and,in many installations, represent a practical limit to the height of theoverall machine.

An object of the presentinvention is to provide an improved heatingprocedure in a false twist yarn texturizing process which permitsincreased production rates without increasing apparatus size.

In accordance with the principles of the present invention, thisobjective is obtained by subjecting the twisted yarn to a sonic flow ofsuperheated steam confined in closely surrounding relation to the yarn.This procedure not only obviates the above-noted limitations of existingelectrical heaters, but differs materially from the steam heatingprocedures heretofore proposed by Finlayson et al. and Kunzle. Thepresent procedure is similar to the Finlayson et al. and Kunzle and theexisting non-contact electrical heaters in that the yarn is heatedprimarily by convection of a surrounding fluid heating medium. Thisbasic mode of heat transfer is inherently more efficient than theconduction heating carried out by contact heaters because it is moreuniform. The procedure of the present invention presents the convectionheating medium to the yarn in the form of a continuously changingrelatively small body of fluid heating medium flowing with a sonicvelocity. In terms of effiCiency,-the convective heat transfercoefficient of turbulent high speed steam against the entire surface ofthe yarn is greater than that of conduction through the interface of theyarn and contact heater surfaces or that provided by the non-turbulentconvection of either Finlayson et al. or the existing non-contactelectrical heaters, or Kunzle. Thus, the present procedure has theadvantage of more efficiently delivering more heat in a shorter periodof time than prior art procedures.

Present machines which have the capability of running at productionrates of ypm require heaters of nearly 40 inches in height. With thepresent heating procedures, the same production rate can be achievedwith a sonic fiow confinement length of 3 7% inches and rates as high as275 ypm can be obtained with only a 6 inch length of sonic flowconfinement.

It is further an object of the present invention to provide an improvedheating procedure in a process of heat setting a false twist inthermoplastic yarn which can be effectively carried out with the use ofsimple low profile equipment, which equipment is economical tomanufacture, operate and maintain.

These and other objects of the present invention will become moreapparent during the course of the following detailed description andappended claims.

The invention can best be understood with reference to the accompanyingdrawings wherein an illustrative device for carrying out the inventionis shown.

In the drawings:

FIG. 1 is a yarn flow diagram illustrating the procedural steps of thefalse twist process to which the present improvement relates and,schematically, the apparatus used in carrying out the process; and

FIG. 2 is a vertical sectional view of the heater structure used incarrying out the improved heating procedure of the present invention.

Referring now more particularly to the drawings, there is shown in FIG.1 thereof a yarn flow diagram including a schematic showing of theapparatus for carrying out the process of the present invention. Asshown, a supply of thermoplastic yarn in the form of a yarn package isprovided from which yarn 12 is drawn, as by a first or upstream set ofnip rolls 14 or the like. The nip rolls l4 serve to continuously feedthe yarn 12 into a texturizing or crimping zone, indicated at 16, fromwhich the yarn is continuously drawn by a second or downstream set ofnip rolls 18 or the like. The yarn 12 passing through the texturizingzone 16 moves past a false twisting device, schematically indicated at20, at a position intermediate to the ends thereof so as to impart andmaintain a twist in one direction, which extends upstream to theupstream nip rolls 14. The twist V turizing thermoplastic yarn by heatsetting a false twist therein is conventional.

The present invention is more particularly concerned with improvementsin the procedure for elevating the temperature of the twisted yarn andthe cooperation of this procedure with the other steps of theconventional process as outlined above. It will be understood that theterm texturizing as used herein is inclusive of the known process ofone-zone draw-texturizing, wherein the yarn is simultaneously drawn andfalse-twisted, as well as the known two-zone draw-texturizing process.It will be further understood that after the yarn 12 passes thedownstream nip rolls 18, it may be subjected to further conventionalprocedures, the diagram of FIG. 1 illustrating simply the subsequentforming of a package 22 of the texturized yarn as by a take-up roll 24.a

In accordance withthe principles of the present invention, the heatingof the yarn in the texturizing process is accomplished by confiningasonic flow of superheated steam in closely adjacent surrounding relationalong a predetermined length of the yarn l2 continuously moving betweenthe upstream nip rolls 14 and the false twisting device 20. FIG. 2illustrates a device, generally indicated at 26, for carrying out theheating procedure. As shown, the device 26 includes a plenum chamber 28having a superheated steam inlet pipe 30 communicating therewith. Itwill be understood that the inlet pipe 30 communicates with a source ofsuperheated steam (not shown) which may be of any conventionalconstruction capable of delivering a continuous supply of superheatedsteam at an accurately controlled temperature and pressure.

Disposed in communication with the plenum chamber 28 is a sonic flowconfining member 32. Formed in the end of the member 32 whichcommunicates with the plenum chamber is a converging nozzle 34 formedthe axis of which is aligned with the axis of the cylindrical surfaceforming the throat 36.

When the device 26 is used to carry out the improved heating procedureof the present invention in the texturizing process previouslydescribed, the device 26 is positioned within the texturizing zone 16between the upstream nip rolls l4 and the false twisting device so thata predetermined longitudinal extent of the yarn continuously movingtherein will pass through the longitudinally aligned throat 36 andpassage 40 provided by the device. The passage 40 is smaller than thethroat 36 so as to establish a continuous flow of superheated steam fromthe plenum chamber 28 through the converging nozzle 34 and outwardly tothe atmosphere through the throat 36.

In accordance with the principles of the present invention, thetemperature and pressure of the source of superheated steam applied tothe plenum chamber 28 are so related to the size of the throat 36 as toestablish a sonic flow of superheated steam through the throat 36. Theterm sonic-flow as used herein relates to a fluid flow velocity equal inmagnitude to the speed of sound waves within the flowing medium. Theseconditions are also interrelated to the longitudinal forces imposed onthe yarn as it passes through the various stages within the texturizingzone 16, the heating stage. The longitudinal forces transmitted to theyarn by the present heating procedure differ from those transmitted byknown heating procedures in that in the present procedure the heatingmedium is in sonic motion rather than being stationary. Thus, with thepresent invention the forces applied to the yarn by the heatingprocedure are susceptible to being reversed in direction by reversingthe orientation of the device 26 with respect to the direction of yarnmovement. On the one hand, where the device 26 is oriented so that thesonic flow of superheated steam within the throat 36 is in the samedirection as the direction of yarn movement, the action of the sonicflow is to tend to pull the yarn from the upstream nip rolls l4 and topush the yarn toward the downstream nip rolls 18. This action tends toincrease the tension of the yarn between the device 26 and the upstreamnip rolls and to decrease the tension of the yarn between the device andthe downstream nip rolls. On the other hand, where the-device 26 isoriented so that the sonic flow of super-heated steam within the throat36 is in'a direction opposed to the direction of yarn movement, theaction of the sonic flow is to tend to push the yarn toward the upstreamnip rolls 14 and to pull the yarn from the downstream nip rolls 18. Thisaction tends to decrease the tension of the yarn between the upstreamnip rolls l4 and the device 26 and to increase the tension of the yarnbetween the device 26 and the downstream nip rolls 18. In either event,the action of the heating medium in sonic movement with respect to theyarn establishes a turbulent environment not presented by other priorart procedures necessitating the maintenance of the yarn in afairly tautcondition as it passes through the heating stage in order to secure goodcrimp stability and prevent undue breakage. For this reason,particularly when texturizing commercially available fully drawn yarnssuch as polyester which exhibit low shrinkage tension during annealing,it is usually desirable to operate with low to even slightly negativeoverfeeds, the preferred range being somewhat variable depending on theyarn speed, in the general range of plus 4 to minus 3 percent overfeed.In general, higher yarn speeds require lower overfeeds because of theincreased breaking tendencies of fast-running slack yarns. Higheroverfeeds are also believed possible with yarns which shrink more thanpolyester during 'texturizmg.

The present process improvements would have applicability with falsetwist processes as combined with drawing processes include bothsequential or two-zonel draw texturizing and concurrent or one-zone drawtexturizing. In the case of one-zone draw texturizing of undrawn feedyarn, the feed ratio of the upstream and down-stream nip rolls may bevaried taking into account the disclosure with respect to feed ratioscontained in British Pat. No. 1,263,055. Where draw-spun partiallyoriented feed yarns are utilized in accordance with the disclosurecontained in the commonly. assigned U.S. Patent application in the nameof Ronald J. Small, Ser. No. 349,930, filed Apr. 11, 1973, the feedratio utilized is varied taking into account the feed ratio formuladisclosed therein.

The direction of sonic flow with respect to the direction of yarn travelhas an effect on the tenacity and crimp development of the processedyarn due to the action noted above. Steam flow in an upstream directionwith respect to the yarn travel as shown inFIG. 2 is preferable becauseof the somewhat better crimp development. However, where greatertenacity with less crimp development is desired in the finished yarn, adownstream flow can be utilized.

With the above in mind, the variables in the heating procedure of thepresent invention can best be understood by first considering the heattransfer characteristics of the heating medium to the yarn and then theheat conduction within the yarn.

In analyzing the heat transfer it is assume 1. that the steam enteringthe nozzle from the plenum chamber is at zero velocity,

2. that the steam expands ideally (no losses) through the convergingnozzle to sonic velocity and travels through the throat with no frictionlosses,

3. that the steam system is operating at steady flow equilibriumadiabatically, that is, the heat loss through the insulated jet body isnegligible compared with the total energy flow,

4. that the turbulent flow of steam around the yarn surface is developedat the converging section of the nozzle andremains constant along thenozzle throat, and

5. that the heat transfer due to radiation is negligible.

For a steady state, adiabatic process the entropy remains constant; thusS S,. An ideal fluid expanding through a converging nozzle has anincrease in velocity with a decrease of the exhaust pressure until sonicvelocity at the throat is reached. The pressure ratio at a sonicvelocity is called the critical pressure ratio and is given by,

P2/P1 /k 1) The heat transfer, q, to an adiabatic process is zero, andthis leaves the change of internal energy Ah, and change of kineticenergy, AKE. Rewriting the equation, Ah AKE 0. With these threeequations and the initial steam conditions, the velocity, temperature,and density of the steam in the jet nozzle can be calculated. Thesurface convection heat transfer coefficient of superheated steam,without condensation, flowing inside pipes is given by McAdams, HeatTransmission, page 172 from experimental data. An equation in terms ofdimensionless ratios was fitted to the data and is N 0.021 (N where theBiot Modulus is, N hD/K, and the Reynolds number is, N V p D/u. Usingthe state of the steam in the jet nozzle, the Reynolds number can becalculated. Thus, the value of the Biot modulus is known, and thesurface convection heat transfer coefficient, h, can be calculated.

In analyzing the heat conduction in the yarn, it is assumed 1. that theyarn is twisted to such an extent that it is treated as a monofilamentfor internal heat conduction,

tofth e jet leng t h divided by the yarn. speed,

3. that the heat conduction equation is solved for the condition of along cylinder subjected to a sudden change in environmental temperature,

4. that the initial temperature distribution in the cylinder (yarn) isuniform and equal to T 5. that at the time 6 0 the yarn is exposed tosteam whose temperature is T and 6. that the convection heat transfercoefficient, h, between the surface of the body and the fluid is uniformand does not change with time.

The yarn is idealized as a long cylinder, and the heat transfer in along cylinder is governed by the basic differential heat equation wherea is the thermal diffusivity} The initial and boundary conditions forthe solution are:

1. that at time 0 0 the cylinder temperature is uniform atT=T 2. that attime 0 0 the cylinder is exposed to a fluid at temperature T and 3. thatthe unit surface conductance, h, between the surface of the body andfluid is uniform and does not change with time. Thus, the heat flow atthe surface is equal to q Ah (T, T The solution to the differentialequation evaluated at these boundary conditions is given indimensionless temperature ratios (Tr/r T)/(T,, T as a function of theFourier modulus, (all/r and the reciprocal of the Biot modulus, (klhrThe solution is given in Principles of Heat Transfer by Kreith, pp.141-142, FIG. 4-10.

From the above, it can be seen that with an increase of only the steampressure, holding the superheated temperature constant, the temperatureand velocity inside the throat remain almost constant. The only changeis the increase of steam density. Thisincrease in density produces anincrease of Reynolds number resulting proportionally in a largerconvection heat transfer coefficient. When this larger coefflcientiscarried through the temperature calculation.'thefinal'result is-adecrease in the temperature difference .of the steam and yarn. Thus, itis seen that the steam pressure governs the heating rate while the inputsteam temperature controls the heater temperature. With this analysis itis possible to determine if the steam jet heater is heating in thetransient zone where yarn temperature will fluctuate or in the stable ornear steady state zone where the yarn temperature is uniform.

Within the parameters of steam temperatures and pressures necessary toobtain a yarn temperature required to heat set the false twist withinthe particular yarn being processed, it is further to be noted that asthe steam temperature is increased, crimp development of the processedyarn increases but the tenacity thereof decreases. The exact temperatureand pressure conditions of the steam utilized are therefore dependentupon the balance of these two characteristics desired in the endproduct.

The process of this invention may be applied in conjunction with any ofseveral standard means of introducing false twist. Spindle twisting isused herein only as an example. As noted before, the overfeed (orunderfeed) which may be used varies with both the nature of the feedyarn and the type and speed of texturizing. A low-shrinkage yarn such ascommercial, drawn, 150- denier polyester may be treated at overfeedsashigh as about 4 percent at conventional 160 ypm production rates. Athigher yarn speeds, breakage resulting apparently from increasedballooning at the twister begins to be observed. In generaLforcommercial drawn polyes-- ter, in speed ranges of 160 to 275 ypm,underfeeds of 0.5 to 1.5 percent are preferred, with about 1 percentbeing optimum. Necessary adjustments for drawtexturizing are discussedhereinbefore.

Another factor in establishing the optimum overfeed, essentially apartfrom the shrinkage factor, is the need to assure good crimp and crimpstabilization. Too taut a yarn cannot twist to its maximum potential andthus falls short of maximum crimp. Too loose a yarn twists withoutadequate frictional control at the twister, and this lack of frictionpermits the twist to slip past at intervals before the crimp becomesheat-set. The balancing and control of these and other operationalfeatures is well-known in the art of false-twist texturizing.

Although current interest in false-twist texturizing lies primarily inlow-shrinkage polyester, the process of the invention is believedequally applicable to other fibers, such as nylon. Operating parametersfor higher shrinkage drawn fibers will obviously need adjustment fromthose for commercial polyester, particularly in' terms of permitting oreven requiring higher ranges of overfeed, ranging as high as about 8percent. Other factors such as yarn diameter, thermal conductivity, andspecific heat will also affect the temperature and dwell time in theheat-setting zone. One skilled in the art, given these principles andthe following specific details for polyester, will readily be able toapply the invention to other polymer types, yarn sizes, and false-twisttex-' turizing systems.

An illustrative example of texturizing multifilament Dacron polyesteryarn (regular, drawn, ISO-denier Type 56) utilizes a steam source at atemperture of 470F and a pressure of 55 psig. At conventional productionrates of 160 ypm with a 1 percent underfeed as previously noted, theheating procedure of the present invention can be carried out utilizinga flow confining member 32 having a throat 36 approximately 3.50

inches long and 0.083 inches in diameter (member 40 being 0.021 inchesin diameter). By following the analysis previously set forth, this steampressure and temperature condition produces a temperature of 346.6F andpressure of 23.3 psig with sonic velocity of 1,682 ft/sec. inside thethroat 36. These throat conditions produce a heating effect whichresults in a calculated yarn surface temperature of 324F and yarn centertemperature of 297F. The temperature differential between the heatingmedium and yarn is 22.6F (346.6 324F) which is within the stable heatingrange.

A production rate of 275 ypm can be obtained utilizing the same steamsupply conditions (470F'and 55 psig) by extending the length of the flowconfining member 32 to obtain the same dwell time of 0.036 seconds. Theamount of length extension required is approximately 2.5 inches makingthe member 32 a total of 6 inches in length. The yarn produced at the275 ypm rate with the conditions indicated above had approximately 3.73gm/denier tenacity,y 18,2 percent elongation, 31.8 percent crimpdevelopment and 5.2 percent shrinkage. The false twist device utilizedwas a conventional Fagg spindlette capable of over 600,000 rpm run at614,000 rpm at the 275 ypm production rate. Still greater texturizingspeeds may be made possible by employment of higher temperatures andlonger jets.

It will be understood that while the improvements of the presentinvention have been disclosed in connection with the production oftexturized yarns having a uniform crimp throughout the longitudinalextent thereof, the underlying principles of the present'invention andthe improved heating procedures thereof have applicability to theproduction of false twist texturized yarns of the novelty configurationsdisclosed and claimed in the commonly-assigned US. Pat. application inthe name of Joe F. London, Jr., Ser. No. 291,586, filed concurrentlyherewith.-

, It thus will be seen that the objects of this invention have beenfully and effectively accomplished. It will be realized, however, thatthe foregoing preferred specific embodiment has been shown and describedfor the purpose of illustrating the functional and structural principlesof this invention and is subject to change without departure from suchprinciples. Therefore, this invention includes all modificationsencompassed within the spirit and scope of the following claims.

I claim:

1. In a process of texturizing thermoplastic yarn I which includes thesteps of continuously feeding yarn into and out of a texturizing zone,maintaining a false twist in yarn within said texturizing zone bycontinuously imparting a twist to the yarn at a twisting position spaceddownstreamfrom an upstream position of restraint, and raising and thenlowering the temperature of the yarn during the continuous movementthereof between said upstream position of restraint and said twistingposition to heat set the twist therein, the improvement which compriseseffecting the raising of the temperature of the yarn by confining asonic flow of superheated steam in closely adjacent surrounding relationalong a predetermined longitudinal extent of the yarn continuouslymoving between said upstream position of restraint and said twistingposition.

2. The improvement as defined in claim 1 wherein the sonic flow ofsuperheated steam is in a longitudinal direction opposed to thelongitudinal direction of movement of the predetermined longitudinalextent of yarn contacted thereby.

3. The improvement as defined in claim 1 wherein the confinement of saidsonic flow of super-heated steam is along a cylindrical surfacethroughout said predetermined longitudinal extent.

4. The improvement as defined in claim 1 wherein said sonic flow isestablished by communicating a source of superheated steam along afrustoconical surface converging with one end of the cylindrical surfaceof confinement.

5. The improvement as defined in claim 1 wherein the sonic flow ofsuperheated steam is in a longitudinal direction the same as thelongitudinal direction of movement of the predetermined longitudinalextent of yarn contacted thereby.

6. The improvement as defined in claim 1 wherein the yarn is made ofpolyester and the feeding of the yarn into and out of said texturizingzone is accomplished with an underfeed of between 0.5 percent and 1.5percent.

7. The improvement as defined in claim 6 wherein the source ofsuperheated steam is maintained at a temperature of the order of 470Fand at a pressure of the order of 55 psig.

8. The improvement as defined in claim 7 wherein the feeding of saidyarn into and out of said texturizing zone is at a rate of approximately275 ypm.

9. The improvement as defined in claim 8 wherein said predeterminedlongitudinal extent is of the order of 6 inches.

10. The improvement as defined in claim 9 wherein the sonic flow ofsuperheated steam is of the order of 1,682 ft/sec.

1. In a process of texturizing thermoplastic yarn which includes thesteps of continuously feeding yarn into and out of a texturizing zone,maintaining a false twist in yarn within said texturizing zone bycontinuously imparting a twist to the yarn at a twisting position spaceddownstream from an upstream position of restraint, and raising and thenlowering the temperature of the yarn during the continuous movementthereof between said upstream position of restraint and said twistingposition to heat set the twist therein, the improvement which compriseseffecting the raising of the temperature of the yarn by confining asonic flow of superheated steam in closely adjacent surrounding relationalong a predetermined longitudinal extent of the yarn continuouslymoving between said upstream position of restraint and said twistingposition.
 2. The improvement as defined in claim 1 wherein the sonicflow of superheated steam is in a longitudinal direction opposed to thelongitudinal direction of movement of the predetermined longitudinalextent of yarn contacted thereby.
 3. The improvement as defined in claim1 wherein the confinement of said sonic flow of super-heated steam isalong a cylindrical surface throughout said predetermined longitudinalextent.
 4. The improvement as defined in claim 1 wherein said sonic flowis established by communicating a source of superheated steam along afrustoconical surface converging with one end of the cylindrical surfaceof confinement.
 5. The improvement as defined in claim 1 wherein thesonic flow of superheated steam is in a longitudinal direction the sameas the longitudinal direction of movement of the predeterminedlongitudinal extent of yarn contacted thereby.
 6. The improvement asdefined in claim 1 wherein the yarn is made of polyester and the feedingof the yarn into and out of said texturizing zone is accomplished withan underfeed of between 0.5 percent and 1.5 percent.
 7. The improvementas defined in claim 6 wherein the source of superheated steam ismaintained at a temperature of the order of 470*F and at a pressure ofthe order of 55 psig.
 8. The improvement as defined in claim 7 whereinthe feeding of said yarn into and out of said texturizing zone is at arate of approximately 275 ypm.
 9. The improvement as defined in claim 8wherein said predetermined longitudinal extent is of the order of 6inches.
 10. The improvement as defined in claim 9 wherein the sonic flowof superheated steam is of the order of 1,682 ft/sec.